Formation of Hard Grey Scale (HGS) on the Surface of a Cold-finger in the Aluminium Production Industry
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The present work has investigated the formation mechanisms of Hard Grey Scale (HGS) in a duct transporting the off-gas from aluminium production pot cells to the Gas Treatment Centre (GTC). For that purpose, a cooled fouling probe or “cold-finger” was designed, built, and used in fouling experiments performed at a Norwegian aluminium production site with durations ranging from several hours to several months, over the course of nearly three years. The effect of HGS growth on heat transfer was studied by means of heat flux sensors placed on the surface of the cold-finger, which allowed monitoring fouling resistance over time and determining the HGS thermal conductivity. This value, which was not reported in the literature, can be used to evaluate the impact of HGS growth on surfaces where heat transfer is of importance (e.g. heat exchangers). Due to the particulate fouling nature of HGS growth, a comprehensive overview of the mathematical principles behind the two main Computational Fluid Dynamic (CFD) modelling approaches, namely Lagrangian and Eulerian, are presently reviewed. Moreover, the unpublished implementation of a Eulerian CFD model for particle deposition developed during the project is described in the final part of the thesis. Opposed to previous hypothesis based on particle recrystallization or presence of a continuous binding phase, strong evidence is presented in this thesis for an HGS formation mechanism based on inertial deposition of small particles in the size range between tens of nanometres to few micrometres. The quenching of electrolyte vapours in the form of atmolite (NaAlF4) was found to be the key component in the formation of HGS by providing the smallest particle fraction able to fill the voids between the larger depositing particles, and thus to produce a closely packed structure responsible for the hardness and high surface-adhesion of HGS. This conclusion was reached by several means, i.e. by: Characterization of the particle size distributions of HGS samples and off-gas dust samples by image analysis and calculation of particle capture efficiencies on the cold-finger surface, with good agreement with other experimental particle deposition studies using different types of particles, as well as with numerical studies based on CFD Models. Compositional analysis of HGS samples showing the enrichment of atmolite in HGS samples by Quantitative X-Ray Diffraction (Q-XRD). Clear particulate nature of HGS with absence of a binder phase and inter-particle sintering observed from Transmission Electron Microscope (TEM) measurements. The HGS growth in the front side of the cold-finger (i.e. upstream side of the off-gas) had a linear relationship with time with an average growth rate of 0.9 mm per month and no signs of growth stabilization within the experimental duration range (i.e. up to eight months). Thermophoretic effects were found to be negligible in HGS growth rates even though they were significant in the formation of fluffy deposits accumulating on the rear side of the cold-finger surface (i.e. downstream side of the off-gas). The overall results of the present study provide with a solid understanding of the physical nature and growth characteristics of HGS. The results can also be used to assess how future changes in the aluminium production process might affect the fouling behaviour in equipment exposed to the off-gas from the pot cells.